A general negative pressure type booster device is configured such that a valve body connected to a movable diaphragm partitioning an interior of a housing into a forward negative pressure chamber and a rear pressure conversion chamber has a shaft hole, a plunger which can be reciprocated in an axial direction with reference to the valve body and moves integrally with an input member and an a valve mechanism including a negative pressure valve performing communication/cutoff between the negative pressure chamber and the pressure conversion chamber depending on reciprocal movement of the plunger with reference to the valve body and an air valve performing communication/cutoff between the pressure conversion chamber and atmospheric air are built in the shaft hole, a reaction member having a rear surface with which a front end portion of the plunger and a front end portion of the valve body can be engaged and an output member having a rear end portion engaged with a front surface of the reaction member and being capable of moving in an axial direction with reference to the valve body are mounted, and the movable diaphragm and the valve body operate with an operation of the input member to operate the output member. The negative pressure type booster device is described in, for example, Japanese Patent No. 3528321.
In the negative pressure type booster device described in Japanese Patent No. 3528321, the movable diaphragm includes an annular rubber diaphragm and an annular metal plate disposed in front thereof, the rubber diaphragm is airtightly held at an annular outer peripheral bead portion formed on an outer peripheral edge of the diaphragm in a housing, and is airtightly fixed on an outer peripheral portion of the valve body together with an inner peripheral portion of the plate on an annular inner peripheral bead portion formed on an inner peripheral edge of the diaphragm. In the rubber diaphragm, between the outer peripheral bead portion and the inner peripheral bead portion, a folded portion having a curved portion on the front side and connected to the outer peripheral bead portion at an outer peripheral portion is formed, and an annular portion connected to an inner peripheral portion of the folded portion and the inner peripheral bead portion is formed.